Admixtures of blowing agents and thermoplastic materials are subjected to elevated temperatures in a mold to produce a thermoplastic article that is foamed and has a cellular structure. The blowing agent makes the thermoplastic article of lower density and lighter in weight. The lower density and weight enables the foamed thermoplastic articles to be used more extensively in applications such as aircraft and space vehicles.
A method currently used to foam thermoplastics is to admix with the thermoplastic the blowing agent and/or other like material which volatilizes and generates gas upon exposure to elevated temperatures. The gas permeates the thermoplastic and produces the internal cellular structure of the foamed thermoplastic article. Many of the blowing agents used are chemical blowing agents which react with the thermoplastic through thermal or chemical decomposition in order to generate the required gas.
High temperature thermoplastic engineering polymers are useful in modern applications because of their high temperature resistance, their ability to be reinforced with fibrous or nonfibrous materials, and their ability to be processed at reasonable temperatures.
In foaming high temperature thermoplastic engineering polymers, conventional blowing agents volatilize at too low a temperature to be highly effective. Upon decomposition within a high temperature thermoplastic, the gas generated by these conventional blowing agents merely escapes the admixture and will not form the desired cellular structure because the thermoplastic will not have reached its melting temperature and cannot flow to form cells. Even when blowing agents which have decomposition temperatures at about the melting point of the high temperature thermoplastics are used other problems, such as lack of oxygen in a closed mold or insufficient gas production, are still present.
Advanced composite sandwich structures are fabricated in a laminated array that has a skin on either side of a core. The core is generally made of a cellular structure, honeycomb or structural foam (thermoset, thermoplastic, or nonpolymeric) material and is fabricated or molded to final dimensions because the machining of core materials is an expensive and labor-intensive process. Further, any dimensional deviation in the core may cause intrusion/extrusion of the bond line at the interface of the core and the skin. Intrusion can produce voids and thick bond lines, and extrusion may develop resin starvation and bridging. All these anomalies are detrimental and cause a decrease in the strength of sandwich structures.
Present methods for fabricating advanced composite high temperature thermoplastic polymeric materials such as those having a melting point temperature over 350.degree. F., are expensive and difficult to use in producing complex geometry components. As an example, to fabricate a sandwich structure, expensive commingled graphite/thermoplastic skins may be used as reinforcements with pre-machined or pre-molded cores. Commingled fabrics are made by the intermingling of graphite and thermoplastic fibers. High pressures and temperatures are needed to produce sound structures, e.g., 10 ksi at 750.degree. F. is required to injection mold solid polyetheretherketone (PEEK), and 350 psi at 750.degree. F. is required to consolidate PEEK preimpregnated and commingled fabric. Due to the high operating and molding pressures needed for production and due to the difference in the thermal coefficients of expansion of the materials, cracks and voids may form in the structure. Also, failure to thoroughly wet the reinforcement may cause a weak interface between the matrix and fiber resulting in fiber pull-out and interfacial cracks and voids.
Dealing with these concerns of the fabrication process is expensive. Indeed according to one estimate relating to composite manufacture for use in helicopter fuselages, fabrication accounts for 58% of fuselage formation cost with the remaining 42% being relatively equally divided between the costs of materials, assembly and quality assurance.
In in-situ fabrication of foamed high temperature thermoplastic advanced composites, high internal pressure is needed to consolidate the sandwich. Due to the high operating and molding pressures needed for production and due to the difference in thermal coefficients of expansion of the thermoplastic and skin, cracks and voids may form in the composite if insufficient pressure is present. Failure to produce sufficient in-situ pressure to consolidate the thermoplastic skin will also result in an inconsistent foaming action.
The present invention is directed to overcoming one or more of the above problems.